1. Field of the Invention
This invention relates to a magnetic sensor system and the use of this system for measuring the liquid level within an opaque ampoule.
2. Description of the Related Art
Fabrication of a metal-oxide-semiconductor (MOS) integrated circuit (IC) involves numerous processing steps. A gate dielectric, typically formed from silicon dioxide ("oxide"), is formed on a semiconductor substrate which is doped with either n-type or p-type impurities. For each MOS field effect transistor (MOSFET) being formed, a gate conductor is formed over the gate dielectric, and dopant impurities are introduced into the substrate to form a source and drain. Such transistors are connected to each other and to terminals of the completed integrated circuit using conductive interconnect lines. Typically, multiple levels of interconnect are needed to provide the connections necessary for a modern, high-transistor-density IC.
Many of the processing steps mentioned above involve deposition of a material layer onto a semiconductor topography (semiconductor substrate with overlying layers and structures). Chemical vapor deposition (CVD) is a commonly used technique for material deposition in semiconductor processing. For example, a gate conductor is typically patterned from a polysilicon layer deposited using CVD. Interlevel dielectrics, which insulate layers of interconnect from each other and from underlying devices, are also generally deposited using CVD. Interconnect formation often involves deposition of metal films by either CVD or by physical vapor deposition (PVD) methods, such as evaporation or sputtering.
CVD methods involve exposing a substrate to gases, known as precursors, which undergo a chemical reaction to form the desired material on the substrate surface. The substrate is generally heated to provide energy both for the chemical reaction to occur, and for the deposited atoms to migrate on the surface of the growing layer and form stable bonds. The desired precursors should react such that the desired material remains on the substrate surface, while remaining precursor components are not incorporated into the deposited film. It is also desirable, for example, that the precursors react to deposit a film only upon the substrate, so that coating of deposition chamber walls and fixtures is minimized. Constraints such as these can make selection of suitable precursors challenging. For some materials, precursors are available in a relatively convenient form. For example, oxide may be deposited using silane and oxygen, or silane and nitrous oxide precursors. These precursors are readily available as bottled gases.
CVD of metals, on the other hand, often requires metal organic sources, many of which are liquid at room temperature. Examples of such liquid metal organic sources are dimethyl aluminum hydride (DM AH), used in CVD of aluminum, and tetrakis (dimethylamino) titanium (TDMAT), used as a precursor for CVD of titanium nitride. Use of liquid sources in CVD typically requires that a gas be bubbled through the liquid so that a vapor of the liquid is formed. The substrate may then be exposed to this vapor, along with any other needed precursors, during the CVD process. The liquid precursors are often enclosed in sea led ampoules for this purpose.
Ampoules typically have an input and an output to the sealed container. The input carries gas into and near the bottom of the sealed container. When liquid is within the sealed container, the gas bubbles through the liquid as it rises toward the top of the sealed container. The gas combines with the liquid to form a vapor such that the upper portion of the sealed container is filled with the vapor. The output carries the vapor out of the sealed container, and, thus, creating a vapor flow output from the ampoule.
Most ampoules used in semiconductor wafer fabrication equipment are sealed containers capable of being pressurized and capable of housing erosive or volatile chemicals. Hence, the ampoule is often made from an opaque material preventing a view of the liquid level within the sealed container. Often there is a need to know the liquid level within the ampoule. Measuring the liquid level within a pressurized and opaque ampoule is often necessary to ensure that the liquid level does not become so low that it prevents the ampoule from functioning properly. For example, semiconductor wafer fabrication facilities often utilize ampoules to contain liquid metal organic precursor (e.g., TDMAT) for CVD processes. Pressurized gas bubbles through the liquid metal organic precursor within the ampoule creating a liquid metal organic vapor output. If too little or no liquid is in the ampoule, then the ampoule will not provide a proper liquid metal organic vapor output for the CVD process.
A current method used in semiconductor fabrication facilities for determining the liquid level within an ampoule is to, first, pressurize the ampoule. Next, the gas volume within the ampoule is measured. Based on the measured gas volume, the volume of liquid remaining within the ampoule is calculated. Then, the liquid level is derived from the estimated liquid volume. This current method is an indirect means for determining the liquid level and liquid amount within the sealed ampoule. Hence, it may inaccurately quantify the liquid volume within the ampoule. Such inaccuracy may cause numerous process and product quality issues. Running out of the proper amount of liquid within an ampoule can result in lack of proper CVD on a semiconductor wafer. Such improper CVD can cause defective products or unacceptable quality levels. In addition, this current method of measuring the liquid level within an ampoule may cause excessive equipment downtime at CVD operations employing this method of liquid level determination because it disrupts the flow of the CVD operations while measuring the gas volume within the ampoule. Downtime for semiconductor wafer fabrication equipment is very costly and should be avoided whenever possible. It would therefore be desirable to develop a method and apparatus for more accurately determining the liquid level within an ampoule. The desired method and apparatus should also allow the liquid level within an ampoule to be measured without disrupting the ampoule's output flow, so that downtime is minimized.